Common welded joints include butt joints, lap-penetration joints, and lap-fillet joints. Laser welding is a method of joining metal components using a focused beam of coherent light to melt adjoining components and allowing the melt to solidify into a joint. While butt joints may be produced by laser welding, they are not always suitable in the aerospace, automotive, and marine industries. Laser welding of lap-penetration joints and lap-fillet joints is more difficult to accomplish. FIGS. 1 and 2 depict the weld region during laser beam welding of a lap-penetration joint in which a laser beam is directed at the region of an interface 2 between components 4 and 6. Relative movement is effected along the interface 2 between the laser beam and the assembly of components. The laser beam may cause a portion of metal in the weld region to volatilize to produce a keyhole 8 bounded by molten metal 10. The keyhole 8 advances with the movement of the laser beam in the direction of the arrow A. Molten metal 10 solidifies behind the advancing keyhole 8 to create a joint between the components.
In practice, production of lap-penetration joints and lap-fillet joints via laser welding is limited. For example, in a lap-penetration joint as shown in FIGS. 1 and 2, it is well established that the width W of the weld at the interface 2 should be equal to or exceed the thickness t of the thinnest of the components being joined. The welding process must be controlled to minimize formation of voids in the welds that are caused by instabilities in the keyhole and/or volatilization of low melting constituents with high partial pressures (e.g., Mg). In addition, laser welding is relatively costly. The laser beam is generally operated at or above 106 W/cm2; efficiency dictates a need to weld at rates of at least 80 inches per minute (ipm) at this power level. With welding components up to 0.1 inch thick, it is possible to produce the required 0.1 inch weld width W at speeds exceeding 120 ipm. The formation of voids can be adequately controlled by use of defocused beams or bifocal optical systems. However, with thicker materials it is progressively more difficult to achieve the required weld width while still maintaining acceptable weld quality and speeds of travel.
With laser welding lap-fillet joints, the welding system must accommodate variations in lateral placement of the laser beam relative to the joint edge and gaps between the components to attain performance comparable to deposits made with gas metal arc welding (GMAW) at rates that justify using costlier laser welding systems.
One option for overcoming the challenges in laser welding lap-penetration and lap-fillet joints is to use beam integrators, focusing optics (mirrors or lenses) with longer focal length or defocused beams. However, to ensure reliable and consistent optical coupling between the laser and components to be joined and to achieve localized melting at the joint, the power output of the laser system must be increased to compensate for the reduction in power density. With sufficient power output, widened welds can be produced in the more placid conduction mode rather than the keyhole mode. Unlike the latter mode, which involves translation of a cavity (or keyhole) along the joining area, the conduction mode is achieved by translating a molten pool of metal along the joining area. By minimizing the violent volatilization of metal within the keyhole, the more placid conduction mode can eliminate the instabilities inherent with the keyhole mode. As a result, the conduction mode minimizes the formation of voids in the laser welds. However, implementing this approach necessitates using very powerful lasers (e.g., 18 KW to 25 KW, depending on the application) and costly laser generating systems, which makes the approach impractical for many industrial applications.
Another approach to increasing the effectiveness of laser welding is described in U.S. Pat. No. 4,369,348 by oscillating the laser beam at frequencies of over 1000 Hz. This very rapid movement of the laser is intended to distribute and time average the intensity of the laser at a frequency greater than the thermal response time of the metal. In this manner, the time averaged intensity of heat experienced by a particular location at the interface between the metal components being joined is greater than the intensity of heat experienced without oscillation. However, operation of a laser beam at oscillation frequencies of over 1000 Hz is difficult and costly. In addition, the only way to implement this approach is to weld in the conduction mode where a continuous molten pool is maintained.
Accordingly, a need remains for a low cost, effective method of laser welding.